This invention concerns an improvement in the selective preparation of two-carbon atom oxygenated hydrocarbons, namely acetic acid, ethanol, and/or acetaldehyde, from synthesis gas. More particularly, the invention concerns the reaction of synthesis gas in the presence of a rhodium-sodium catalyst under heterogeneous reaction conditions correlated to produce such two-carbon atom products.
The preparation of hydrocarbons and oxygenated hydrocarbons from synthesis gas (essentially a mixture of carbon monoxide and hydrogen) has received extensive study and has achieved commercial adoption. Reaction conditions generally involve temperatures on the order of 150.degree.-450.degree. C., pressures from atmospheric to about 10,000 psig, and hydrogen-to-carbon monoxide ratios in the range of 4:1 to about 1:4, with an iron group or a noble metal group hydrogenation catalyst.
One serious disability of most synthesis gas processes for the production of oxygenated hydrocarbons containing two or more carbon atoms has been the non-selective or non-specific nature of the product distribution. Catalysts which possess acceptable activity generally tend to give a wide spectrum of products, for example, hydrocarbons and oxygenated hydrocarbons having a broad distribution of carbon atom contents. This not only complicates the recovery of desired products, but results in the wastage of reactants to commercially uninteresting byproducts.
United Kingdom Pat. No. 1,501,892 is directed to a process for selectively preparing a mixture of two-carbon atom oxygenated compounds, namely, acetic acid, ethanol and acetaldehyde, using a rhodium catalyst. United Kingdom Pat. No. 1,501,891 describes a process for favoring the production of ethanol relative to acetic acid and acetaldehyde by incorporating iron into the rhodium-based catalyst. U.S. Pat. Nos. 4,096,164, 4,014,913 and copending application Ser. No. 841,054 filed Oct. 11, 1977, concern themselves with enhancing the productivity and/or varying the distribution of the aforementioned two-carbon atom oxygenated compounds by the addition of elements such as molybdenum and/or tungsten, manganese, and thorium and/or uranium, respectively, to the rhodium-based catalyst.
Rhodium catalysts supported by carriers containing sodium, among other materials, have been used for the production of oxygenated hydrocarbons from synthesis gas. In the aforementoned United Kingdom Pat. No. 1,501,892, Table V describes the various rhodium catalysts used in the experiments of the patent, several of which catalysts are shown to have been supported on Davison 59 silica gel. The chemical composition of such Grade 59 silica gel is provided in Grace Technical Bulletin IC-773-15 which indicates the presence of 0.10 weight % Na.sub.2 O (corresponding to 0.074 wt. % Na) in the gel. A lower sodium value for gels or this type is disclosed in Grace Technical Bulletin 202 (1958) at page 2, where a sodium level of 0.02 weight % is indicated to be typical of Davison silica gels. In The Journal of Catalysis, Vol. 54, pp. 120-128 (1978), Bhasin et al. disclose synthesis gas reactions with rhodium catalysts supported on Davison Grace 59 silica gel. At page 121, Table 1 of this publication, the sodium content of the silica gel support is reported to be 0.085 weight %. Thus, the sodium content of silica gel supports of the type used in conjunction with the rhodium catalysts described in the aforementioned United Kingdom patent and the Bhasin et al disclosure is reported in the technical literature to be as low as 0.02 weight % and as high as 0.085 weight %.
An undesirable feature of the synthesis gas process disclosed in the aforementioned patents and The Journal of Catalysis publication is the production of methane along with the desired two-carbon atom oxygenated hydrocarbons. In general, the greater the efficiency of the reaction to methane, the lower the production of the desired oxygenated hydrocarbons. Since methane is a significantly less valuable commercial chemical than either ethanol, acetic acid or acetaldehyde, methane formation is economically undesirable. Moreover, inasmuch as carbon monoxide and hydrogen, the reactants in these processes, are generally produced commercially by steam reforming of methane, the formation of methane, in turn, from synthesis gas necessarily detracts from the overall process efficiency.